JP6427991B2 - Dust core - Google Patents

Description

  The present invention relates to a powder magnetic core obtained by pressure-molding amorphous soft magnetic powder.

  Amorphous soft magnetic alloy containing iron as a main constituent element has smaller hysteresis loss than iron-silicon alloy and has a higher saturation magnetic flux density than iron-silicon-aluminum alloys represented by Sendust. Applications as a magnetic core for power supply in a frequency range of 1 k to 100 kHz are expanding. A dust core formed by mixing a soft magnetic alloy powder with a binder material or the like forms a magnetic gap at the boundary surface between the powders and exhibits a magnetic resistance against an external magnetic field. For this reason, the dust core has a characteristic that the change in the magnetic permeability with respect to the change in the external magnetic field is small, that is, the direct current superposition characteristic is excellent, as compared with the core in which the soft magnetic thin plate materials are laminated. In general, it is effective to increase the relative density of the soft magnetic powder contained in the core in order to improve the DC superposition characteristics of the dust core. In the case of an amorphous soft magnetic alloy mainly composed of iron, a technique for pressing a powder sample at a high density has been energetically studied.

  To increase the relative density of powder cores mainly composed of iron, methods such as lowering the binder content to increase the binding, increasing the molding pressure, and adjusting the powder particle size are proposed. Has been. Considering the influence of the powder particle size on the relative density, the idea of increasing the relative density of the powder magnetic core by mixing and pressing a relatively coarse powder and a fine powder is the patent. Documents 1-3. By mixing the coarse powder and the fine powder, the particle size distribution of the powder has a structure having two peaks, and by filling the gap between the coarse powder with the fine powder, It is thought that the relative density increases compared to the above. On the other hand, in addition to the increase in relative density, it is considered that the magnetic anisotropy derived from the shape of the powder affects the direct current superposition characteristics of the dust core.

JP-A-5-299232 JP 2004-349585 JP JP 2009-54615 A

However, there has been no example of finding a method for improving the DC superposition characteristics by systematically examining and considering the characteristics of both the relative density increase and the powder shape effect in the dust core.
This invention is made | formed in view of said point, The place made into the objective is to provide the powder magnetic core which can improve a direct current | flow superimposition characteristic notably.

  The present invention systematically examines the characteristics of both the increase in relative density by controlling the particle size distribution of amorphous soft magnetic powder and the magnetic anisotropy effect by controlling the shape of the powder. A technique to improve the DC superposition characteristics of dust cores is proposed.

  The dust core of the present invention that solves the above problems is a coarse powder that is an amorphous soft magnetic powder having an average particle size of 50 μm to 120 μm and an aspect ratio of 1 to 6, and an average particle size of 1 μm. It is characterized by being composed of a mixed powder obtained by mixing fine powder which is an amorphous soft magnetic powder having an aspect ratio of 4 or more and 15 or less and having an aspect ratio of 30 μm or less.

  According to the present invention, the gap between the coarse powders is densely filled with the fine powder having a large aspect ratio, so that the direct current superimposition characteristics can be remarkably improved using the effect of the demagnetizing field. Therefore, it is possible to obtain an amorphous powder magnetic core having excellent direct current superposition characteristics. Problems, configurations, and effects other than those described above will be clarified by the following description of the embodiments.

The schematic diagram of the powder distribution in the powder magnetic core with a low relative density. The schematic diagram of the powder distribution in the powder magnetic core with a high relative density. The graph which shows the relationship between a demagnetizing factor and an aspect ratio. SEM image of amorphous powder obtained by mechanical grinding. The initial magnetization curve of a comparative example and an Example.

  An important magnetic characteristic of the dust core is that the magnetic permeability is stable and high when an external bias magnetic field is applied. It is known that the permeability of the soft magnetic core is highest when there is no external magnetic field (at zero bias) (initial permeability), and the permeability gradually decreases as the external magnetic field increases. The decrease in magnetic permeability of the dust core when an external magnetic field is applied varies depending on the type of soft magnetic powder, the relative density of the soft magnetic powder, the magnitude of the magnetic resistance between the powder particles, and the like. By adjusting the structure of these powder magnetic cores, maintaining the magnetic permeability as high as possible when a large external magnetic field is applied is an important characteristic required for the powder magnetic core.

  The feature of the powder magnetic core of the present invention is that the gaps between the coarse powders are densely filled with flat fine powder having an aspect ratio of 4 or more, and the DC superposition characteristics are improved by utilizing the effect of the demagnetizing field. can do.

  Flat or needle-like soft magnetic particles vary in the magnitude of the demagnetizing field formed in the powder depending on the direction. The demagnetizing field in the direction parallel to the longitudinal direction of the particles is very small and the demagnetizing factor takes a value close to 0, but the demagnetizing field in the direction perpendicular to the longitudinal direction is very large and the demagnetizing factor is 1. Take a close value. On the other hand, when the soft magnetic particles are spherical, the demagnetizing field is isotropic, and the demagnetizing factor is 1/3 in all directions of the particles. When flat or needle-shaped soft magnetic particles are arranged so that the longitudinal direction is parallel to the external magnetic field, the generated demagnetizing field is very small, but when the longitudinal direction of the particles is arranged perpendicular to the external magnetic field. Increases resistance to an external magnetic field due to a demagnetizing field.

  When the relative density of the powder magnetic core is low, the gaps between the coarse powders increase, so that the longitudinal direction of the flat or needle-like fine powder filling the gaps is basically distributed in a random direction. FIG. 1 shows a schematic diagram of a powder distribution in a cross section of a dust core having a low relative density. The vertical direction in FIG. 1 corresponds to the direction in which a load is applied during molding.

  On the other hand, when the relative density of the dust core increases, the gap between the coarse powders narrows, and the longitudinal direction of the fine powder tends to be arranged parallel to the surface of the coarse powder. FIG. 2 shows a schematic diagram of the powder distribution in the cross section of the dust core having a high relative density. The vertical direction in FIG. 2 corresponds to the direction in which a load is applied during molding. The “relative density” in the present invention refers to “a ratio (volume ratio) of amorphous alloy powder in the entire powder magnetic core”. Amorphous alloy powder is the sum of coarse powder and fine powder.

  In the portion where the surface of the coarse powder is perpendicular to the external magnetic field direction, the longitudinal direction of the fine powder filling the gap between the different coarse powders is arranged perpendicular to the external magnetic field. As a result, the effect of the demagnetizing field in the gaps between the coarse powders is increased, so that the magnetic saturation of the dust core is suppressed and the DC superposition characteristics are improved.

  When the aspect ratio of the fine powder is 4 or more, the effect of suppressing magnetic saturation due to the demagnetizing field increases. Assuming that the shape of the fine powder is a disk-like flat ellipsoid, the relationship between the demagnetizing factor and the aspect ratio in the direction perpendicular to the disk surface is shown in Fig. 3 from the data of Bozorth (Ferromagnetism, Nostrand, Princeton, 1951). Is given as a graph. In FIG. 3, when the aspect ratio reaches 4, the demagnetizing factor increases to 0.7, and when the aspect ratio exceeds 4, the demagnetizing factor increases slightly moderately. Therefore, the aspect ratio of the fine powder is preferably 4 or more. On the other hand, the aspect ratio is not particularly required for the shape of the coarse powder.

  There are no particular restrictions on the method for producing amorphous coarse powder and fine powder. As a coarse powder manufacturing method, it is possible to obtain an amorphous foil body by using a molten metal quenching method with a high-speed rotating roll, and then pulverize it by a mechanical grinding method such as a vibration mill, a ball mill, or a disk mill. It is. Moreover, it can also obtain by the method of sieving the powder obtained by the water atomization method with a high cooling rate.

  As a method for obtaining a flat fine powder having an aspect ratio of 4 or more, for example, when an amorphous foil is pulverized by a mechanical process, a flat fine powder is obtained as a cutting piece by devising a pulverization condition. Obtained simultaneously with coarse powder. A flat fine powder can be obtained by sieving the powder after pulverization with a mesh, or by separating the powder into a coarse powder and a fine cutting powder with a cyclone classifier or the like. The aspect ratio of the flat fine powder is preferably 4 or more, but it is industrially difficult to obtain a powder having an aspect ratio exceeding 15 in a process using mechanical grinding. For this reason, the aspect ratio of the flat fine powder is preferably in the range of 4 or more and 15 or less.

  The aspect ratio of the coarse powder varies depending on the production process, but is 1.0 to 2.5 when manufactured by water atomization, and 1.0 or more when mechanically pulverizing the amorphous foil. The aspect ratio is preferably in the range of 0 or less.

  As for the particle size of the coarse powder, 90% or more by mass ratio is distributed in the range of 40 μm to 150 μm, and the average particle size (D50) is preferably in the range of 50 μm to 120 μm. When the coarse powder contains particles of less than 40 μm in an amount of 10% or more, it is not preferable because the effect of the flat fine powder separately adjusted and mixed is hindered and the direct current superimposition characteristic is lowered. Further, when containing more than 10% of particles exceeding 150 μm, it is not preferable because a decrease in relative density and a decrease in DC superposition characteristics due to a decrease in moldability occur. The average particle size of the coarse powder can be determined by evaluating the particle size distribution by a sieving method using a mesh, or by directly observing the size of each powder with an optical microscope or an electron microscope. At the time of observation with a microscope, the maximum length of each observed powder is evaluated as the particle diameter.

  The average particle size (D50) of a flat fine powder having an aspect ratio of 4 or more is preferably 30 μm or less, and more preferably 20 μm or less. When the particle diameter exceeds 30 μm, the gaps between the coarse powders are not sufficiently filled, and the magnetic flux density and the direct current superimposition characteristic are reduced. On the other hand, since a flat powder having an average particle size of less than 1 μm and an aspect ratio of 4 or more is industrially difficult to manufacture, the average particle size of the flat fine powder is preferably 1 μm or more. The evaluation of the average particle diameter of the fine powder has a problem that mesh clogging occurs in the sieving method. Therefore, it is preferable to directly observe the size of each powder using an optical microscope or an electron microscope. At the time of observation with a microscope, the maximum length of each observed powder is evaluated as the particle diameter.

  Strictly speaking, as a method for obtaining the aspect ratio of a flat fine powder, it is necessary to evaluate after obtaining a three-dimensional shape of each fine powder. However, since this method is difficult to apply to a large number of fine powders, in the present invention, as a simple evaluation method, 1 g or more of the powder is embedded in a resin, polished, and the cross section of the powder is observed with an optical microscope. The aspect ratio was evaluated from the cross-sectional shape of the powder. In each powder cross section of the optical microscope structure, the maximum length and the minimum length of the powder were measured, and the value obtained by dividing the maximum length by the minimum length was defined as the aspect ratio of each powder. The aspect ratio of 200 or more individual powders was measured by setting the observation area of the embedded sample to an area of 500 μm × 500 μm or more, and the average value was adopted as the aspect ratio of the powder.

  The chemical composition of the amorphous soft magnetic alloy used as a raw material for the dust core preferably includes Fe as a main component and B and Si as essential elements. Other elements may contain 5% or less Cr, Mn, C by mass ratio, and inevitable impurities mixed in in the manufacturing process.

  After mixing the amorphous coarse powder and the flat fine powder at a predetermined ratio, pressure molding is performed using a mold or the like to obtain a soft magnetic core. Before the pressure molding, a binder material for improving the binding property, a lubricant for reducing friction with the mold, etc. may be added to the amorphous soft magnetic mixed powder as necessary. Thus, a dust core having high density and excellent mechanical properties such as strength can be obtained. In the amorphous mixed powder before adding the binder, the lubricant, etc., the proportion of the flat fine powder is preferably in the range of 3 to 50% by mass ratio. When the addition amount of the flat fine powder is less than 3%, the DC superposition characteristics are not improved due to insufficient filling at the boundary of the coarse powder and the demagnetizing effect at the boundary, which is not preferable. When the addition amount of the flat fine powder exceeds 50%, the relative density of the powder magnetic core is lowered, and the direct current superimposition characteristic is lowered.

  The mold temperature during molding can be room temperature, but the relative density can be increased further by molding the mold to 100 ° C or higher with a heater and softening the binder and lubricant. It is. When molding by heating the mold, it is necessary to appropriately set molding conditions in consideration of changes such as softening and vaporization of the binder and lubricant at high temperatures. When forming by heating and holding the mold to a high temperature exceeding 300 ° C, the amorphous powder itself undergoes plastic deformation due to softening, and the gaps between the powders are more densely filled and the relative density increases. . For molding at a high temperature exceeding 300 ° C., it is necessary to select a binder material with high heat resistance in order to prevent the binder material from disappearing due to vaporization.

  The dust core after molding reduces hysteresis loss by heat treatment for the purpose of strain removal. The heat treatment temperature is generally lower than the temperature at which crystallization of the amorphous alloy occurs, and on the other hand, it is preferable that the heat treatment be performed as high as possible. When the molding is performed by heating the mold, the heat treatment temperature is preferably higher than the mold temperature. The relative density of the dust core after the heat treatment is preferably 80% or more, since the saturation magnetic flux density can be increased and the magnetic permeability at zero bias can be increased to 80 or more. When the relative density of the dust core is less than 80%, the saturation magnetic flux density is lowered, and the permeability at zero bias is lowered to less than 80, which is not preferable.

According to the dust core of the present invention, the gap between the coarse powders is densely filled with the fine powder having a large aspect ratio, so that the magnetic resistance of the gap between the coarse powders can be increased. Compared with the magnetic core, the direct current superposition characteristics can be remarkably improved.
The effects of the present invention will be described below through examples.

(Example 1)
In the trial production of an amorphous soft magnetic alloy, Fe, B, and Si are used as raw materials and blended at a mass ratio of 92Fe-5.5Si-2.5B. A 25 μm foil alloy was obtained. Subsequently, the foil body alloy was pulverized by a high-energy mill. An example of SEM observation of the morphology of the pulverized powder is shown in FIG.

  Plate-like flat particles presumed to be fine cut pieces are confirmed on and around the surface of coarse crushed powder having a size exceeding 50 μm. By processing the pulverized powder in solution, coarse crushed powder and flat fine powder are separated, and after drying, sieving with mesh is performed on each, and the average particle diameter D50 = 63μm and the particle diameter is 40μm To 120 μm of amorphous coarse powder, and an amorphous and flat fine powder having an average particle diameter D50 = 12 μm and a particle diameter of 25 μm or less. The flat fine powder was subjected to resin embedding and cross-sectional polishing, and optical microscope observation was performed. As a result, it was confirmed that the average aspect ratio of the cross-section was 6.8. The aspect ratio of the coarse powder was 4.2.

The above-mentioned amorphous coarse powder and flat fine powder are mixed at mass ratios of 9: 1, 8: 2, and 7: 3, respectively, and the binder material PEEK resin is mixed with these amorphous mixed powders. Were mixed at a volume ratio of 9: 1 and stirred while heating at around 400 ° C. to obtain a composite powder of amorphous metal powder and resin. 1.6 g of these composite powders were molded at 400 ° C. and 1600 MPa using a hot press to obtain a ring-shaped molded body (core) having an outer diameter of 13 mm and an inner diameter of 8 mm. The ring core was heat treated at 400 ° C or higher in a nitrogen atmosphere for 1 hour (1 hour) to reduce strain, and then toroidal wound with copper wire and loss per volume Pc (kW / m ³ , by AC BH measuring machine, 0.1 T, 10 kHz), and the incremental permeability when a 10 kA / m bias magnetic field was applied by an LCR meter.

  Table 1 shows the measurement results of the relative density, the loss per volume, and the incremental permeability of the dust core. Nos. 2 to 8 are the present invention materials, and Nos. 1 and 9 to 11 are comparative materials. The magnetic permeability when no bias magnetic field was applied (at zero bias) showed values in the range of 80 to 160 for all samples in Table 1.

  Nos. 1 to 4 in Table 1 show the dust cores when the amount of fine flat powder added is 0, 10, 20, or 30% by mass in the amorphous mixed powder before the PEEK binder is added. The measurement results of relative density, loss per volume, and incremental permeability are shown.

  The relative density of No. 1 (comparative material) that does not contain fine flat powder is 83.5%, and the relative density increases by 1 to 3% for No. 2, 3, and 4 (material of the present invention) that contain fine powder. . In Nos. 2, 3, and 4, it is considered that the relative density increased because the fine powder filled the gaps of the coarse powder. There is no significant difference in the value of loss per volume between No. 1 and No. 2, 3, and 4, but the value of incremental permeability when applying a 10 kA / m magnetic field is No. 1. Compared to No. 2, 3 and 4, the results were about 6-8. In Nos. 2, 3, and 4 of the present invention, it was shown that the incremental magnetic permeability can be increased without increasing the loss by adding a flat fine powder to the coarse powder.

  Next, by adjusting the process conditions when pulverizing the foil body alloy with a high energy mill, by continuously peeling flat pieces from the surface of the foil body, the average particle diameter D50 = 10 μm and the particle diameter is 25 μm or less Thus, fine flat particles having an average cross-sectional aspect ratio of 4.8 were produced. The evaluation results of the dust core when No. 5 to No. 7 in Table 1 and 4.8 fine flat powder of 4.8 are added to the coarse powder by 10, 20, and 30% by mass are shown.

  In each of the samples No. 5 to 7, the same amount of flat fine powder as No. 2 to 4 is added. The relative densities and loss values of Nos. 5 to 7 are almost the same as those of Nos. 2 to 4, and the value of the incremental magnetic permeability is slightly low. From this result, it was shown that when a fine powder having an aspect ratio of 4.8 was added to the coarse powder, the DC superposition characteristics were improved as compared with the sample containing only the No. 1 coarse powder.

  Invention material No. 8 in Table 1 shows the result of adding 50% by mass of fine powder having an aspect ratio of 4.8 in the same amount as coarse amorphous powder. The relative density of No. 8 was 82.3%, the lowest in Table 1, and the incremental magnetic permeability was 33.8, which was lower than those of other inventive materials No. 2-7. In No. 8, it is presumed that the relative density and magnetic flux density were reduced as compared with other materials of the present invention due to the excessive amount of fine powder, leading to a decrease in incremental magnetic permeability.

  The material with the same chemical composition as the foil body produced by the rapid cooling described above is finely ground using a water atomization process and sieved, so that the average particle size D50 is 9μm and the particle size is in the range of 20μm or less. Amorphous powder was obtained. The result of embedding this powder with resin, polishing the cross-section and observing it with an optical microscope confirmed that the shape was close to a sphere with an aspect ratio of 1.3. The evaluation results of the dust core when the fine amorphous particles having an aspect ratio of 1.3 are added to No. 1 coarse amorphous powder by 10, 20, or 30% by mass are shown in Table 1. Shown in 11. The relative densities and loss values of Nos. 9 to 11 were almost the same as those of Nos. 2 to 7, but the incremental permeability values were 30 to 32, which were lower than those of the present invention material. It was found that the effect of improving the DC superimposition characteristics was small when particles close to a sphere with a small aspect ratio were used for filling between coarse powders, although this contributed to an increase in relative density. When adding fine particles close to a sphere having a low aspect ratio, it is presumed that the increase in the incremental magnetic permeability is suppressed by reducing the effect of the demagnetizing field as compared with the fine particles having a high aspect ratio.

(Example 2)
By adjusting the process conditions when grinding with a high energy mill using the amorphous foil body alloy with a thickness of 25 μm prototyped in Example 1 as a raw material, the particle size is 25 μm or less, and the aspect ratio of the cross section is average 8.9 fine flat particles were produced. The fine powder having an average aspect ratio of 8.9 in an amorphous coarse powder having an average particle diameter D50 = 63 μm and a particle diameter distributed in the range of 40 μm to 120 μm, also described in the production method of Example 1. After adding 20% by mass, hot pressing and heat treatment were performed under the same conditions as in Example 1 to obtain a ring-shaped dust core. This sample is designated as invention material No. 12. The relative density of No. 12 was 86.2%, and the incremental permeability when applying a bias magnetic field of 10 kA / m was the highest value of 39.7.

  FIG. 5 shows magnetization curves obtained from magnetic measurements of No. 1, No. 3 and No. 12 ring cores in Table 1. The graph is plotted with magnetic flux density B (T) on the vertical axis and magnetic field H (A / m) on the horizontal axis in FIG. No. 1 contains no flat fine powder, and No. 3 and No. 12 contain 20% by mass of flat fine powder, but the aspect ratio of the fine powder is different. Compared to No. 1 in which the core is formed only with coarse powder, No. 3 and No. 12 containing flat fine powder have a larger slope of the graph on the high magnetic field side, and the decrease in permeability is suppressed. I understand that. In addition, No. 12 with a larger aspect ratio of fine powder has a higher magnetic permeability on the higher magnetic field side than No. 3, so increasing the aspect ratio of fine powder is effective in improving incremental magnetic permeability. I was able to confirm.

  Although the embodiments of the present invention have been described in detail above, the present invention is not limited to the above-described embodiments, and various designs can be made without departing from the spirit of the present invention described in the claims. It can be changed. For example, the above-described embodiment has been described in detail for easy understanding of the present invention, and is not necessarily limited to one having all the configurations described. Further, a part of the configuration of an embodiment can be replaced with the configuration of another embodiment, and the configuration of another embodiment can be added to the configuration of an embodiment. Furthermore, it is possible to add, delete, and replace other configurations for a part of the configuration of each embodiment.

Claims (3)

A coarse powder which is an amorphous soft magnetic powder having an average particle size of 50 μm to 120 μm and an aspect ratio of 1 to 6; and an average particle size of 1 μm to 30 μm and an aspect ratio of 4 to 15 Consists of mixed powder mixed with fine powder which is amorphous soft magnetic powder,
The ratio of the fine powder occupying the mixed powder Ri 30 mass% or less der least 10 mass% in mass ratio,
The amorphous soft magnetic alloy used as a raw material for the amorphous soft magnetic powder is mainly composed of Fe, B, Si as essential elements,
The dust core according to claim 1, wherein the coarse powder is a pulverized powder of a foil body alloy .   2. The dust core according to claim 1, wherein the ratio of the amorphous alloy powder in the entire dust core is 84.3% or more by volume ratio.   3. The powder compact according to claim 1, wherein a particle size of the coarse powder is 90% or more in a mass ratio distributed in a range of 40 μm to 150 μm, and particles exceeding 150 μm are less than 10%. core.

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